An image senor is a sensor converting light into an electric signal. Representative image sensors include APS (active pixel sensor) and PPS (passive pixel sensor) using CMOS. The photo diode used in the image sensor accumulates incident light and converts the accumulated light into an electric signal. The general photo diode outputs a low current from a small amount of light and thus has to lengthen the exposure time in order to accumulate sufficient electric charge for processing the signal. Accordingly, it is difficult to apply an image sensor using the general photo diode to a high-speed camera. Moreover, since a small amount of light is incident in a dark surrounding, images generated using this kind of image sensor have an unreliable quality. Although there have been studies about a unit pixel of an image sensor having high sensitivity, the afterimage effect or the memory effect still remains to be resolved.